The observations mentioned above place important constraints on the formation and the evolution history of QSOs at high redshift and represent a major challenge for theoretical models
(see, e.g., the recent review by [1]}).
The main question is how these extremely massive and luminous sources form and evolve during the first billion year of cosmic history (\(z>6\) ), soon after the formation of the first (pre-galactic) structures in the Universe (\(z\sim 20-30\) ).
